1. Field of the Invention
The invention relates to a system for installing a collimator in a gamma camera. It is intended for use particularly in medical imaging. A gamma camera is an assembly of equipment: detector, stand, and console, enabling nuclear medicine examination to be performed. The collimator of a gamma camera is fixed to the detector.
2. Discussion of the Background
A gamma camera is conventionally used as follows. Patients subjected to examination on such apparatus have a tracer containing a radioactive isotope injected into their bodies. The isotope fixes itself preferentially in a particular organ depending on the type of tracer injected. The function of the gamma camera is to form the image of the plane projection of the gamma radioactivity fixed by said organ, thus revealing its functional state. The image is obtained by detecting gamma radiation coming from the organ under examination and emitted in a single direction so as to project the concentration of the isotope in the organ in said direction. A gamma camera thus comprises a gamma radiation detector constituted, in particular, by a large-area scintillator crystal covered with a network of photomultipliers. When a gamma ray passes through the scintillator, it produces local scintillation which is detected by a plurality of photomultiplier tubes situated in the vicinity of the scintillation. The photomultipliers provide electrical current proportional to the received scintillation. This makes it possible to locate the impact of gamma rays in the scintillator crystal by evaluating the center of gravity of the signals delivered by the photomultipliers.
A device called a "collimator" situated immediately in front of and against the detector is used to select for detection only those gamma rays that are emitted in a single direction, e.g. a direction perpendicular to the inlet face of the scintillator.
In outline, a collimator is an absorbent plate, generally made of lead, which is pierced by a multitude of ducts that pass gamma rays only over a very small solid angle whose axis is parallel to the desired projection direction for the image of radioactive isotope distribution that is to be produced.
In practice, collimators are designed as a function of the energy of the gamma radiation emitted by the isotope injected into the patient and as a function of the desired transparency and spatial resolution characteristics that are to be obtained. In general terms, spatial resolution and transparency depend on the size of the solid angle of the collimator ducts, and it is clear that these two characteristics vary in opposite directions. Depending on the types of examination performed using nuclear medicine, use is made of substances that emit gamma rays at different energies, or else specific resolution and transparency characteristics are required for the collimators. Under such circumstances, a gamma camera is generally fitted with a family of collimators having different geometrical characteristics. In routine operation of the gamma camera, it must be possible to swap the collimator associated with the detector of the camera for a particular examination with another collimator at the request of the operator performing the examination.
The thickness, or in practice the weight, of the collimator depends on the energy of the radiation to be detected and on the looked-for solid angle characteristics. In addition, detectors are generally cantilevered out from a bracket. The unbalance due to the cantilevered detector is normally compensated by counterweights disposed in the mechanism for moving or handling the detector. Since it is not easy to change these counterweights, it has become the practice to associate peripheral masses with lightweight collimators (collimators for low-energy radiation), thereby ensuring that they weigh as much as heavier collimators. When the working weight of the collimators may lie in the range 20 kg to 60 kg, this technique consists in providing collimators which, together with their associated masses, always weigh about 60 kg.
Collimators of such a weight cannot be handled without mechanical equipment. In practice, the gamma camera detector is contained in a metal case in the form of a large pot provided with a rabbet or groove for receiving the collimator. The collimator constitutes a kind of lid for the detector and the case. Installation is based on the principle of lifting the collimator by applying force from below on the outside face of the collimator when said face is turned face-down. In such handling, the collimator is applied to the detector from below after previously turning over the detector. For example, two handling arms mounted on a carriage bear against said outside face from below and disengage the collimator from its storage position. The carriage is then displaced into the vicinity of the gamma camera. To install the collimator, given that the detector is upsidedown with the opening to its case at the bottom, its "lid" is applied thereto by raising the two handling arms. Since this operation is performed from below, it is performed blind.
The collimator is in the form of a relatively thick slab. With a round-field detector, the collimator is naturally round. Round collimators are fixed as follows. A circular groove is formed in the edge of the collimator plate and surrounds the plate. When the collimator is installed to close the case, this groove comes level with a system of screws fixed to the case. The tips of the screws are engaged into the groove by tightening the screws manually. Once the parts are engaged together, the carriage and its handling arms may be withdrawn.
Given that it must be possible for the gamma camera to be used at any angle in three dimensions, and in particular that it must be possible for it to be above the patient, operators responsible for installing collimators are in the habit of tightening the collimator fixing screws very firmly. This avoids any risk of the collimator becoming detached since it could seriously injure a patient by falling. This assembly technique suffers from three drawbacks. Firstly the system is unhandy since it has to be operated blind at the moment the collimator is installed. Given that installation takes place from below, the operator cannot see at exactly which moment the carriage is properly in place, and this makes it difficult to center the collimator relative to the opening in the case. Secondly, tightening the screws excessively can sometimes make it difficult to lossen them again when the collimator is to be replaced.
Further, the use of round sealed gamma cameras is presently being replaced by the use of gamma cameras in which the examination field is square or rectangular. Under these conditions, centering becomes even more difficult since for mechanical reasons the collimator is also rectangular and needs to be inserted into a rectangular opening. This is difficult to do when acting blind and from below.
There exists a second solution which is also applicable to collimators for a round field gamma camera. In such cameras, the collimators possesses retractable handles that can be grasped by a sufficiently sturdy operator who then approaches the opening in the gamma camera case while it is upwardly directed. The operator then lowers the collimator at arms' length onto the opening and rotates it through a quarter turn to engage studs mounted on the edge of the collimator or on the case into bayonet fittings mounted on the corresponding portion of the case or the collimator. This method of fixing can naturally only be used for lightweight collimators, and because of the quarter turn, it is unfortunately usable only with round collimators.
A third technique exists in which a carriage engages a collimator by means of hooks and places the collimator onto a gamma camera case from above. Once the collimator has been placed on top of the case, screws are engaged through the collimator so as to be received in the solid portion of the case. The heads of the screws enable the collimator to be held in place when the gamma camera is turned over so that the outside face of the collimator faces the ground. The drawback presented by this technique relates largely to the need to tighten a large number of screws, thereby wasting operator time. In addition, as in the preceding cases, the screws may be overtightened out of concern for safety, such that subsequent loosening can become difficult.
In addition, there is another problem that arises: this is the problem of moving the collimator from the cupboard in which it is stored to the gamma camera that is to receive it while ensuring sufficient safety to prevent the collimator escaping from the manipulator arms of the carriage.